Summary:In order to evaluate the risk of cytomegalovirus (CMV) associated disease after allogeneic stem cell transplantation (SCT), 158 consecutive patients at risk for infection were analyzed. BMT was performed in 101 patients and peripheral blood stem cell transplantation (PBSCT) in 57 patients. CMV antigenemia was found in 57 cases (56%) after BMT and 27 cases (47%) after PBSCT, respectively. CMV antigenemia resistant to a 14-day course of GCV was found in 26 patients (26%) after BMT but in only four patients (7%) after PBSCT (P Ͻ 0.01). Eighteen patients (11%) developed CMV disease, 14 post BMT and four post PBSCT. Lethal CMVrelated interstitial pneumonia (CMV-IP) occurred in 13 cases of whom 12 patients were bone marrow recipients (P = 0.04). The subgroup of seronegative patients with a CMV seropositive donor had a significantly lower risk of developing CMV antigenemia, GCV-resistant CMV antigenemia (P Ͻ 0.01) and CMV-related disease (P = 0.01). In conclusion, the incidence of persistent CMV antigenemia and CMV-IP was significantly reduced when allogeneic transplantation was performed with peripheral blood stem cells instead of bone marrow. These findings suggest that our previous in vitro data on improved immune reconstitution after allogeneic PBSCT as compared to allogeneic BMT have clinical relevance. Bone Marrow Transplantation (2000) 25, 665-672. Keywords: CMV infection; CMV interstitial pneumonia; pp65 antigenemia; BMT; PBSCT; immune reconstitution Cytomegalovirus (CMV)-related disease is one of the most frequent infectious complications after allogeneic stem cell transplantation (SCT). 1,2 Despite new therapeutic options, symptomatic CMV infection still has a high mortality rate, especially when interstitial pneumonia (CMV-IP) develops. 3,4 Well-known risk factors of CMV infection and related diseases are acute graft-versus-host disease (aGVHD) and positive serology for CMV of recipient and/or donor before transplantation. 1,3 There is still controversy about an increased incidence of CMV infection or CMV-related disease after allogeneic bone marrow transplantation (BMT) with a matched unrelated donor (MUD) instead of an identical sibling donor (ISD). 5,6 The transmission of CMV infection from seropositive blood products can be neglected since white blood cell filtration is routinely used. 3,7,8 Prophylaxis with ganciclovir (GCV) given to all seropositive patients leads to a decreased incidence and severity of CMV infection but this therapeutic option is associated with neutropenia and in consequence bacterial and fungal infections are more frequent in these patients. 9,10 Pre-emptive therapy based on screening for culture-proven CMV infection, CMV antigenemia by leukocyte expression of pp65 matrix protein, or CMV-DNA by PCR testing reduces the incidence of CMV diseases. 11-19 Viral load appears to be proportional to CMV antigenemia in the systemic circulation. Thus, CMV antigenemia-triggered preemptive treatment lowers the duration and side-effects of antiviral therapy after SCT which is considered ...